The invention is related to the field of high electron mobility transistors (HEMTs), and in particular to a high electron mobility transistor possessing transport properties of a GaN HEMT with the normally-OFF character of a GaN MOSFET with high breakdown voltage.
Nitride-based transistors will play a very important role in solving the energy challenge in the near future. Specifically, power transistors made of these materials are expected to be enabling devices for advanced transportation systems, more robust energy delivery networks and many new revolutionary approaches to high-efficiency electricity generation and conversion. All these systems rely on very efficient inverters to step-up or step-down electric voltages. For example, in hybrid vehicles power transistors with blocking voltages in excess of 1,000 V are needed to step-up the voltage from the batteries to the voltage required to operate the engine. Nowadays, these devices are made of semiconductors such as Si or SiC, however the limited breakdown voltage of Si and the poor mobility of SiC make commercial devices currently available very bulky, heavy and inappropriate for future generations of hybrid vehicles. Nitride devices offer unsurpassed potential high-efficiency power electronics demanding large high-voltages and low ON resistances.
In spite of the tremendous potential of nitride semiconductors in high efficiency power applications, no, commercial device currently exists. However, three different devices are under consideration at the research level. FIG. 1A shows a schematic diagram of a horizontal AlGaN/GaN high electron mobility transistor (HEMT) 2. The transistor 2 includes a gate structure 8 being formed on a AlGaN barrier layer 16. A GaN channel layer 14 is positioned beneath the barrier layer 16. The source 12 and drain 10 are formed in parallel with the barrier layer 16 and channel layer 14. Although this transistor 2 has shown excellent ON resistance and voltage blocking capabilities, the large channel charge densities induced at the interface between the AlGaN barrier layer 16 and GaN channel layer 14 make the fabrication of normally-OFF devices extremely challenging.
FIG. 1B shows a vertical HEMT structure 4. The vertical HEMT structure 4 includes an AlGaN barrier layer 32 formed beneath a gate structure 18. A GaN channel layer 24 is positioned beneath the barrier layer 32 and the drain 26 of the HEMT structure 4. Two source elements 20, 22 are used to define the source of the HEMT structure 4. These source elements 20, 22 are positioned on a dielectric layers 28, 30. The dielectric layer 28,30 are placed within an etched region of the channel layer 24. The vertical HEMT 4 shown in FIG. 1B requires a smaller area than the horizontal HEMT 2 however it is also difficult to fabricate reliable normally-OFF devices.
Another commonly known transistor is the GaN metal-oxide-semiconductor field effect transistor (MOSFET) 6, as shown in FIG. 1C. The GaN-MOSFET 6 includes a gate structure 34 being positioned on a dielectric layer 46. The dielectric layer 46 is formed on a p-type GaN channel layer 44. The drain 36 of the MOSFET 6 is positioned on a n-type GaN layer 40. The n-type GaN layer 40 is formed in a doped region of the p-type GaN channel layer 44. The source 38 is formed on an n+-type GaN layer 38. The n+-type GaN layer 38 is formed in a doped region of the p-type GaN channel layer 44. The GaN metal-oxide-semiconductor field effect transistors (MOSFETs) are the preferred option for normally-OFF devices however the poor transport properties of the inverted channel significantly increases the ON resistance of these devices. None of the three options currently being pursued for power GaN devices is able to combine the high voltage, low ON-resistance and normally-OFF conduction required by the power electronics